Study On Water And Heat Characteristics Of Snow And Freezing-thawing Black Soil Under Different Snow Cover

As the most common and important upper boundary conditions in the seasonal frozen area of Northern China in the freezing and thawing period of soil, snow cover may directly affect the distribution and the migration process of soil water and heat. Through field test, snow and frozen soil internal water and thermal characteristic parameters and relevant meteorological factors were observed in the area of seasonal freezing and thawing soil in Harbin city under four different mulching conditions, bare land(BL), natural snow(NS), compacted snow(CS) and the thickness of the snow(TS), respectively. Using the advanced mathematical statistics, comparative analysis, grey system theory analysis method, the variation law of internal water and heat characteristics of snow and frozen soil under different snow cover pattern has been deeply explored. The simulation of snow hydrological factors by using the SNOW-17 model and the global sensitivity analysis of the parameters of model output were conducted. The main conclusions are as follows:(1) The liquid water content of internal snow was related to snow depth and age. The middle layer of snow is the secondary stable area of liquid water content. The greater the depth of snow, the vertical distribution difference of liquid water content of the internal snow is larger. Properly compaction has little effect on amplitude of the liquid water content on snow. When the amount of snow cover is at a certain level, the main driving factors of snow melting are atmospheric temperature and solar radiation. The difference of the final melting day of snow with different depths and densities is small. Snow density and snow depth have a weaker effect on the ablation rate of snow. In the horizontal direction, the heat flux variation trend of each snow layer is nearly constant, whereas in the vertical direction, the absolute value of the snow heat flux increases from the surface layer to bottom snow layer. The variability of the snow layers of the heat fluxes are above the moderate degree, and the variability from the surface layer to the bottom layer has an increasing-decreasing tendency. The coefficient of variation of snow heat flux in the snow bottom layer is the minimum, which demonstrates that the heat e xchange between the snow and frozen soil interface is the weakest. The strong variability of snow heat flux in the 10-20 cm snow profile showed that the snow layer heat flux is affected by shortwave solar radiation at this snow depth, and the possibility of phase transition of the snow was increased. The snow input energy is less than the output energy in the stable snow period. In the horizontal direction, the heat flux variation trend of each snow layer is nearly constant, whereas in the vertical directio n, the absolute value of the snow heat flux increases from the surface layer to bottom snow layer. The variability of the snow layers of the heat fluxes are above the moderate degree, and the variability from the surface layer to the bottom layer has an increasing-decreasing tendency. The coefficient of variation of snow heat flux in the snow bottom layer is the minimum, which demonstrates that the heat exchange between the snow and frozen soil interface is the weakest. The strong variability of snow heat flux in the 10-20 cm snow profile showed that the snow layer heat flux is affected by shortwave solar radiation at this snow depth, and the possibility of phase transition of the snow was increased.(2) The snow surface is affected by the radiation of the snow-air interface, and the temperature of atmosphere near the ground is an active variation zone of the snow temperature. The middle layer of the snow cover is a stable region of temperature change. The bottom snow layer is the secondary active zone of the snow temperature because of the weak heat transfer of the snow-soil interface. When the amount of snow reaches a certain level, the compaction treatment increases the temperature of the bottom of the snow. The increasing snow density can increase the snow internal temperature variation, changing the thermal state of snow cover significantly. Air temperature, net radiation, wind speed, water vapor pressure and humidity are the main factors that affect the snow temperature. The fitting degree of the principal component and snow layer temperature decreased with increasing snow depth. The fitting degree of the main meteorological factors and the regression equation of the surface temperature of the snow cover were better than that of snow temperature in the middle and bottom layers.(3)The dramatic degree of total soil moisture changes in bare land was larger than that of the snow-covered plots. This difference was due to the high albedo, low thermal conductivity, and high heat capacity of snow. Thus, the snow reduced the input of net radiation and retarded the energy exchange between the land and the atmosphere. Consequently, the snow cover significantly changed the freezing speed of the soil in this seasonally frozen area and affected the distribution of the soil moisture in the vertical direction. Different snow cover types led to differences in the distribution of snow properties. Specifically, the snow cover types controlled the energy exchange between the snow and soil, thereby affecting the distribution of the soil temperature potential. Consequently, the soil moisture accumulation was very different both temporally and spatially. The temporal and spatial variations of liquid soil moisture intuitively describe the solid-liquid phase change process in this seasonally frozen region. Snow cover changes the distribution of the soil temperature in the lower layers of the snow, thereby affecting the phase change of the soil moisture and leading to temporal and spatial distribution differences in the liquid soil moisture content. Snow cover effectively restrained the temporal and spatial variation amplitudes of the liquid soil moisture content. Specifically, the phase change of the underlying surface soil was affected most significantly by compac ted snow. The differences in total soil moisture caused by different snow cover treatments in the majority of samples were greater than or equal to the significant level(P<0.05). These differences demonstrate that the snow cover type played an important role in regulating the total soil moisture. However, the snow cover type did not affect the total soil moisture at all soil depths. The scope of the impact was limited, and the different snow cover treatments exerted a greater influence on the total soil moisture content of shallow soil than on deep soil. The feedback regulation capacity of the total soil moisture in the shallow layers was clearly influenced by the snow cover treatments to a greater extent than the deep soil layers. Snow cover has a greater regulatory effect on the variation of liquid moisture content more than the variation of total soil moisture content. The snow compaction treatment had the strongest effect on soil moisture preservation in the shallow soil layer, whereas the bare land treatment exhibited the poorest soil moisture preservation. In the same soil profile, the total soil moisture was 4.91% to 7% higher in CS than in BL. In conclusion, the snow compaction treatment had a significant influence on improving the liquid soil moisture content in the deep soil.(4) The snow under different snow cover treatments modulates the energy exchange between the underlying surface and atmosphere by different albedo and turbulent fluxes. During the freezing period, the snow cover treatment significantly improves the soil temperature and has a positive effect of heat preservation. The thermal insulation effect is in the order of TS>CS>NS, and the difference between the four treatments is significant(P<0.05). When the amount of snowfall is at a certain level, according to the existing snow parameterization scheme, snow compaction treatment increases the amount of net radiation transmission in the snow and further improves the heat preservation effect of the snow cover on the soil. Because of the double barrier effect of low heat conductivity and high albedo characteristics on the heat of the snow-air interface and snow-soil interface, deeper snow absorbed more heat and had superior thermal insulation properties. The lan d surface of the four different plots is bare during the melting period. Thus, the boundary conditions are consistent. The difference of the surface temperature is not significant(P=0.1141>0.05).(5) Snow will inhibit the phase transformation degree of so il, affect the rate of phase change of soil. Snow and snow equivalent density play an important role on the the high value zone’s development time of soil phase. Soil thermal difference under different snow cover treatments is extremely significant. The snow cover pattern played a important role on the variation of the heat of soil phase change. The heat of soil phase change in the shallow layer under compacted snow treatment is the largest.(6) Under different rainfall intensities, the snow correction parameter- SCF played decisive role on the model output; UADJ was not sensitive to the two output variables; SCF, MBASE, MFMAX, and MFMIN were always the sensitive factors of the top four of the SWE output value; the three parameters MFMAX, MFMIN and TIMP were negatively correlated with the model output, while SCF, NMF, MBASE and PLWHC were positively correlated with the model output; the model parameters are sensitive to the output under small precipitation intensity; the intensity of small precipitation can improve the sensitivity of other parameters, which is beneficial to the effective identification of the model parameters.In summary, based on Heilongjiang Province, we investigated the related issues of water and heat characteristics in the snow-frozen soil system and simulation of snow hydrological factors in Songnen Plain hinterland’s seasonal frozen soil area, which will not only provide support for the research of soil water and heat migration in this region, enrich water and heat transfer mechanism of the freezing and thawing black soil, but also provide a new land cover pattern for agricultural production in Heilongjiang Province and a theoretical basis for rational utilization of agricultural water resources.